Generally, the present invention relates to optical communications systems, and more particularly to Raman fiber amplifiers and methods of operating same.
Optical communications systems frequently employ optical amplifiers to amplify the communications signal as it propagates along an optical fiber link. One type of amplifier that finds frequent use is the erbium-doped fiber amplifier (EDFA). However, the EDFA uses a special fiber rather than the span fiber and the gain spectrum of the EDFA is far from uniform, thus resulting in nonuniform gain across the spectrum of a wavelength division multiplexed (WDM) signal. This is particularly a problem as the bandwidth of WDM signals increase with the addition of more channels. Gain flattening filters (GFF) have been introduced to the fiber link to increase loss for those wavelengths that see high gain in an attempt to provide more uniform gain across the WDM spectrum. However, gain flattening filters are inefficient, since they only increase the net system losses. Another problem with EDFAs is that spontaneous emission from the inverted gain medium introduces noise to the signal.
First order Raman amplifiers are becoming increasingly more common, and are often used in conjunction with EDFAs. First order Raman amplifiers avoid some of the problems of the EDFA. The Raman amplification takes place in the fiber optics link itself, and there is no requirement that a length of special fiber be spliced into the link. Furthermore, since there is no population inversion, Raman amplification is a relatively low noise process.
A number of problems remain, however, with first order Raman amplifiers. For example, despite the fact that Raman amplification is an inherently low-noise process, noise may still be transferred from the Raman pump to the signal, and the low noise characteristics of Raman amplification have not yet been fully realized. This is particularly the case where second order Raman amplification has been used to amplify the first order Raman light.
In addition, it is desirable to increase or improve the overall noise figure performance in the communications link. Improving the noise figure performance of the amplifiers in the communications link allows less amplifiers to be used per length of fiber span, which reduces the need to convert the optical signal to electrical signal as often. Reducing the number of optical-to-electrical conversions, greatly affects the cost benefit of the system. Further, increasing the noise performance figure reduces the launch power needed for the optical signal in the link, which avoids certain fiber non-linearities that affect signal propagation performance.
Therefore, there is a need for improved second order Raman amplifiers for use in optical communications fiber links that provide sufficient amplification without introducing unacceptable levels of noise. Furthermore, there is a need to reduce the inefficient use of gain flattening filters and increase the noise figure performance in fiber communications systems.
In accordance with the teachings of the present invention, a Raman optical amplifier is disclosed for amplifying an optical signal propagating in an optical fiber associated within an optical communications system. In one embodiment, the amplifier includes a first Raman pump source including a first pump laser coupled to the fiber to provide first order Raman pump light for amplifying the optical signal, and a second Raman pump source including a second pump laser coupled to the fiber to provide second order Raman pump light for amplifying the first order Raman pump light. The first and second order Raman pump light is introduced into the fiber in either a co-propagating or counter-propagating direction relative to the propagation direction of the optical signal in the fiber. Other Raman pump sources can also be employed to generate first order, second order or third order Raman pump light propagating in either the co-propagating or counter-propagating direction in the fiber for additional amplification. The second pump laser may include first and second pump laser elements that generate the second order Raman pump light, where a center wavelength of the light generated by the first laser element is different than the center wavelength of the light generated by the second laser element. The second pump laser may also include more than two pump laser elements that generate second order pump light at different wavelengths.
The amplifier can include other features. For example, at least one of the pump power and pump light output spectrum of the first pump source can be adjustable and respond to changes in at least one of signal channel loading and Raman pump level of the second Raman pump source. Further, the first Raman pump source can receive a control signal from a controller indicative channel loading conditions. Also, an EDFA can be included in the fiber. The pump output spectrum of the first Raman pump source is controlled so as to provide Raman gain and the fiber having a spectral non-uniformity that at least partially compensates for gain spectral non-uniformity of the EDFA.